This invention relates to oscillators and more particularly to monolithic astable circuits used in oscillators in which the frequency of oscillation is controlled by the series resonance of a piezoelectric crystal.
As is well known, an astable multivibrator is a circuit that generates a periodic non-sinusoidal signal, and the circuit does not need to be triggered. Prior art circuits of astable transistor multivibrator circuits usually contain a pair of transistors and a pair of equal valued capacitors one of which connects the collector of the first transistor of the pair with the base of the second transistor and the other one connects the collector of the second transistor to the base of said first transistor, and a pair of equal-valued resistors which are used to bias the bases of the first and second transistors. The resistance-capacitance combination indicated provides the time constant for the period of the non-sinusoidal signal. Where discrete components are used, the periodicity of the non-sinusoidal signal can be reasonably accurately reproduced. However when such arrangements are to be reduced to integrated circuit form, i.e., a monolithic circuit arrangement, neither the resistor nor capacitor value can be derived with a high degree of accuracy. Further, such an arrangement is not conducive to the use of a crystal for stabilization of the periodicity of the circuit.
One of the problems in building a stable oscillator circuit is to control the loop gain. This is particularly true where a crystal oscillator circuit is involved, because the circuit should then be designed so that the loop gain is always greater than one where the crystal is operating in the series-resonance mode. This is necessary to insure that the oscillator starts and runs properly. However, it should be noted that the loop gain should be less than one at any spurious frequency at which the crystal may be resonant.
It is an object of this invention to provide an astable crystal controlled circuit in which the loop gain is well controlled, and in which the loop gain may be readily calculated.
It is a further object of this invention to provide a monolithic astable circuit in which the frequency of operation is determined by the series resonance of a piezoelectric crystal.
it is another object of the invention that the effective resistance of the circuit at the spurious frequencies would always be much higher than the resistance at the fundamental frequency.